1. Field of the Invention
The present invention relates to an electro-photographic image forming apparatus using a two-component developer. More particularly, the invention relates to a toner density estimating method and to an apparatus for maintaining a uniform toner density in a developing unit. The invention is further directed to a toner supplying method, an apparatus using the toner density estimating method and to an apparatus to maintain a uniform toner density.
2. Description of the Related Art
In an image forming apparatus using a two-component developer, the toner density in a developing unit is maintained in the range of 6% to 10% so as to form an image having a uniform density. One method of maintaining a uniform toner density is to mount a toner density sensor in the developing unit. Since the toner density sensor is expensive, another method has been developed without using the toner density sensor. In this method, the number of dots of an output image is calculated to measure the amount of toner consumed, and toner is supplied to the developing unit according to the measured toner amount. However, since the amount of toner consumed varies with the developing voltage, temperature, humidity, and various other external factors in addition to the number of dots, errors may occur in the measurement of the amount of toner consumed.
In order to solve this problem, another method has been developed to estimate the toner density in the developing unit. A test pattern is formed on an intermediate transfer belt (ITB) to be used to measure the toner density, the photo-reflectance of the test pattern is measured and the toner density in the developing unit is estimated using the measured reflectance of the test pattern. The test pattern is a toner image having a predetermined length and width formed to allow the toner density to be estimated. This method does not need a toner density sensor, and can control the toner density relatively accurately according to the density of the test pattern. However, when the density of the test pattern varies due to changes in temperature, humidity, charging voltage, or other external factors in addition to the variation of the toner density in the developing unit, the cause is difficult to identity. Therefore, the toner density in the developing unit may be measured as being outside its setting range, even when it is not. In this case, toner for maintaining the density of the test pattern is erroneously supplied, causing toner to be over-supplied or under-supplied. This can cause additional problems such as scatter of toner, leakage of the developer, and increase in the driving torque of the developer.